In ITU-R (International Telecommunication Union Radio communication sector), call for proposals of IMT-Advanced radio interfaces has started, and standardization works have started aiming to completing specifications in 2011.
With that, in 3GPP (3rd Generation Partnership Project), study of LTE-Advanced is starting as an advanced system of LTE Release 8 (to be referred to as Rel-8 LTE hereinafter).
In this kind of communication systems, one or more resource blocks (RB) are assigned to a user apparatus both in downlink and uplink communications. Resource blocks are shared by multiple user apparatuses in the system. The base station apparatus determines a user apparatus to which a resource block is to be assigned from among a plurality of user apparatuses every subframe that is 1 ms, for example. The subframe may also be called a transmission time interval (TTI). The determination of assignment of radio resources is called scheduling. In downlink, the base station apparatus transmits a shared channel using one or more resource blocks to a user apparatus selected in the scheduling. This shared channel may be called a physical downlink shared channel (PDSCH). In uplink, a user apparatus selected in the scheduling transmits a shared channel to the base station apparatus using one or more resource blocks. This shared channel may be called a physical uplink shared channel (PUSCH).
When scheduling of radio resources is performed, it is necessary to signal (or report) assignment information indicating which user apparatus is assigned the shared channel basically every subframe. A downlink control channel used for this signaling is called a physical downlink control channel (PDCCH) or a downlink L1/L2 control channel (DL-L1/L2 Control Channel). The PDCCH, for example, includes the following information (see, for example, non-patent document 1):                Downlink scheduling information        Uplink scheduling grant        Acknowledgment information (ACK/NACK: Acknowledgment/Negative-Acknowledgment information)        Transmission power control command bit        
The downlink scheduling information may include information regarding a downlink shared channel. More specifically, the downlink scheduling information may include downlink resource block assignment information, identification information of user apparatus (UE IDs), the number of streams, information regarding precoding vectors, data sizes, modulation schemes, and information regarding hybrid automatic repeat request (HARQ).
The uplink scheduling grant includes, for example, information regarding an uplink shared channel. More specifically, the uplink scheduling grant may include uplink resource assignment information, identification information of user apparatus (UE IDs), data sizes, modulation schemes, uplink transmission power information, and information regarding a demodulation reference signal used in uplink MIMO.
The acknowledgement information (ACK/NACK) indicates whether retransmission is necessary for PUSCH transmitted in uplink.
In uplink, the PUSCH is used to transmit user data (i.e., a normal data signal). Also, separately from the PUSCH, a physical uplink control channel (PUCCH) is provided to transmit, for example, a downlink channel quality indicator (CQI) and acknowledgement information (ACK/NACK) for the PDSCH. The CQI is used, for example, for scheduling and adaptive modulation and channel coding (AMC) of the physical downlink shard channel. In uplink, a random access channel (RACH) and signals indicating assignment requests for uplink and downlink radio resources may also be transmitted as necessary.
By the way, the acknowledgement information (ACK/NACK) can be essentially represented only by 1 bit, but, the acknowledgement information has the most basic function in retransmission control, and exerts large effects on system throughput. Therefore, it is desirable that the acknowledgement information (ACK/NACK) is fed back immediately after it occurs. In addition, as mentioned above, CQI indicates channel state of downlink, and is basic information in scheduling and adaptive modulation and channel coding and the like. Since the channel state may change every moment, it is also desirable that CQI is frequently fed back to the base station.
When a resource block is assigned for uplink data transmission, these pieces of control information can be reported to the base station apparatus immediately by using the resource block. However, there is a case in which it is necessary to report acknowledge information or CQI for the downlink data channel to the base station apparatus even though such resource block is not assigned.
In the system of the LTE scheme, the single-carrier scheme (SC-FDMA scheme) is adopted in the uplink, and the system is devised such that the acknowledgement information or CQI can be immediately reported to the base station apparatus in the above-mentioned case. On the other hand, from the viewpoint of further improving use efficiency of radio resources and improving data rate, the multicarrier scheme is preferable rather than the single-carrier scheme.